What you are adding is not subtrim. It's just trim. Subtrim is all about getting your servos centered properly before you start trimming your plane. It is also handy on transmitters with analog trims to get everything right electronically and fly with all the levers centered.

Now for your issue, without seeing your plane I would guess you have a wing incidence issue. In the case of needing a lot of up elevator that would indicate too low an incidence angle compared to the horizontal stabilizer. Before you go changing it though, you need to understand that there are 3 different adjustments that affect pitch issues. CG, wing and stab incidence, and thrust line. Since the stab is permanently glued in, there's not much point in trying to adjust it although that may well be the cause of your problem. How exactly are inverted and knife edge flight very difficult? If you are needing a lot of down elevator to maintain inverted flight, you may still be nose heavy, but a glide test should let you eliminate that. If the plane tucks in knife edge (I would expect toward the canopy if it's an incidence issue) getting all 3 right will certainly reduce it but may not eliminate it. Before messing with the incidence, make sure the CG is right to give you a fairly shallow glide until you start stalling, then test the thrust line (there should be no pitch change when going from full throttle to idle or vice versa). Only then can you set the wing incidence, but then that will probably require you to go back and adjust the other items.